1. Field of the Invention
The present invention relates to a multilayer optical recording medium having three or more recording layers.
2. Description of the Related Art
Optical recording media such as compact discs (CDs) and digital versatile discs (DVDs) are widely utilized as information recording media. In recent years, attention has also been given to optical recording media that utilize blue or blue violet laser light as the irradiation light so that a larger amount of information can be recorded than that available using convention laser light systems.
Optical recording media are generally classified into three types: a ROM (Read Only Memory) type in which data cannot be recorded or rewritten, an RW (Rewritable) type in which data can be rewritten, and an R (Recordable) type in which data can be recorded only once.
In the R and RW type optical recording media, data is recorded by irradiating a recording layer with laser light to form recording marks that have a reflectivity different from that of surrounding space areas. Data is typically reproduced from an optical recording medium by irradiating a recording layer with laser light and detecting the difference in reflectivity between the recording marks and the space areas with a photodetector.
Such optical recording media can be provided with a plurality of recording layers, therefore resulting in higher recording capacities. When recording data onto an R or RW type optical recording medium that has a plurality of recording layers, the laser light intended for recording can be adjusted and focused upon a target recording layer to be recorded so that the data may be recorded on the target recording layer selectively. Laser light intended for reproduction can also be adjusted and focused upon a target recording layer to be reproduced so that data may be reproduced from the target recording layer selectively. Furthermore, in a multilayer recording medium having three or more recording layers, it is preferable that the recording films of the respective recording layers have extinction coefficients of not higher than 0.5 in view of the transmittance and the recording sensitivity thereof, excluding the L0 layer which is the farthest recording layer from light incident surface and thus need not transmit laser light.
Such R and RW type optical recording media that have a plurality of recording layers are preferably configured so that the recording layers have substantially the same recording sensitivities, and therefore, the optimal recording powers of the laser light for recording information on the respective recording layers are substantially the same.
When a recording layer arranged far from the incident surface of the laser light is irradiated with the laser light, the recording layers lying closer to the incident surface of the laser light than the farthest recording layer from light incident surface cause absorption and reflection. Therefore, the laser light reaches the recording layer with an accordingly lower intensity. The farther the recording layer is, the lower the recording sensitivity becomes. Consequently, when recording information on a recording layer arranged farther, or, in particular, on a recording layer arranged farthest away from the incident surface of the laser light, it is necessary to apply a laser light having a higher recording laser power than that of the laser light used when recording information on recording layers lying closer to the incident surface of the laser light than the recording layer lying farther away. This problem becomes even more profound when the number of layers, or recording layers in particular, is three or more.